The differentiation of ischemic recoverable myocardium from irreversibly injured tissue is difficult with existing techniques. Myocardial perfusion imaging with thallium-201 with injection during ischemia has been advocated for this purpose, based on the appearance of redistribution. Recent studies identified a significant number of patients with ischemia, however, who have minimal or no redistribution on their serial perfusion images. An alternative approach to the delineation of acutely ischemic myocardial tissue utilizes changes in metabolic substrate preference. While studies can be done with positron tomography using radiolabeled glucose analogs to identify ischemic myocardium, this technique is not suitable for use in most hospitals that have only conventional Anger type scintillation cameras. An alternative approach, suitable for use with Anger cameras, can detect changes in the relative concentration of a modified fatty acid and thallium in zones of ischemia. The volume of distribution of fatty acids is markedly increased in ischemia, and preliminary studies in our laboratory suggest that dual tracer imaging of the distribution of a modified fatty acid, 3- methyl-p-[123I]-phenylpentadecanoic acid (which has a long myocardial residence time), and thallium-201, can be used to identify ischemic tissue. Normal zones of myocardium have a similar regional distribution of modified fatty acid and thallium, zones of acute ischemia have a relative excess of both radiopharmaceuticals. The proposed studies will compare the behavior of modified fatty acid/thallium to that of 14C labeled 2-deoxyglucose in experimental animals with varying degrees of ischemia, and will define the prognostic value of the modified fatty acid/thallium ratio in patients with wall motion abnormalities scheduled for revascularization.